Update config files docs

This commit updates the docs for the config files to explain the new
mechanism for customizing the configuration directory via the
environment variable CONF_DIR.

Relates #25990

docs/reference/migration/migrate_6_0/packaging.asciidoc

@@ -15,10 +15,9 @@ packages.
 Previous versions of Elasticsearch enabled setting `path.conf` as a
 setting. This was rather convoluted as it meant that you could start
 Elasticsearch with a config file that specified via `path.conf` that
-Elasticsearch should use another config file. Instead, `path.conf` is now a
-command-line flag. To start Elasticsearch with a custom config file, use `-c
-/path/to/config` or `--path.conf /path/to/config`. Here, `/path/to/config` is
-the *directory* containing the config file.
+Elasticsearch should use another config file. Instead, to configure a custom
+config directory, use the <<config-files-location,`CONF_DIR` environment
+variable>>.
 
 ==== Default path settings are removed
 

docs/reference/setup/configuration.asciidoc

@@ -9,26 +9,42 @@ The configuration files should contain settings which are node-specific (such
 as `node.name` and paths), or settings which a node requires in order to be
 able to join a cluster, such as `cluster.name` and `network.host`.
 
+[[config-files-location]]
 [float]
-=== Config file location
+=== Config files location
 
-Elasticsearch has two configuration files:
+Elasticsearch has three configuration files:
 
-* `elasticsearch.yml` for configuring Elasticsearch, and
-* `log4j2.properties` for configuring Elasticsearch logging.
+* `elasticsearch.yml` for configuring Elasticsearch
+* `jvm.options` for configuring Elasticsearch JVM settings
+* `log4j2.properties` for configuring Elasticsearch logging
 
-These files are located in the config directory, whose location defaults to
-`$ES_HOME/config/`.  The Debian and RPM packages set the config directory
-location to `/etc/elasticsearch/`.
+These files are located in the config directory, whose default location depends
+on whether or not the installation is from an archive distribution (`tar.gz` or
+`zip`) or a package distribution (Debian or RPM packages).
 
-The location of the config directory can be changed with the `path.conf`
-flag, as follows:
+For the archive distributions, the config directory location defaults to
+`$ES_HOME/config`. The location of the config directory can be changed via the
+`CONF_DIR` environment variable as follows:
 
 [source,sh]
 -------------------------------
-./bin/elasticsearch --path.conf /path/to/my/config/
+CONF_DIR=/path/to/my/config ./bin/elasticsearch
 -------------------------------
 
+Alternatively, you can `export` the `CONF_DIR` environment variable via the
+command line or via your shell profile.
+
+For the package distributions, the config directory location defaults to
+`/etc/elasticsearch`. The location of the config directory can also be changed
+via the `CONF_DIR` environment variable, but note that setting this in your
+shell is not sufficient. Instead, this variabled is sourced from
+`/etc/default/elasticsearch` (for the Debian package) and
+`/etc/sysconfig/elasticsearch` (for the RPM package). You will need to edit the
+`CONF_DIR=/etc/elasticsearch` entry in one of these files accordingly to change
+the config directory location.
+
+
 [float]
 === Config file format
 

docs/reference/setup/install/deb.asciidoc

@@ -197,7 +197,7 @@ locations for a Debian-based system:
 | conf
   | Configuration files including `elasticsearch.yml`
   | /etc/elasticsearch
-  | path.conf
+  | <<config-files-location,CONF_DIR>>
 
 | conf
   | Environment variables including heap size, file descriptors.

docs/reference/setup/install/rpm.asciidoc

@@ -185,7 +185,7 @@ locations for an RPM-based system:
 | conf
   | Configuration files including `elasticsearch.yml`
   | /etc/elasticsearch
-  | path.conf
+  | <<config-files-location,CONF_DIR>>
 
 | conf
   | Environment variables including heap size, file descriptors.

docs/reference/setup/install/zip-targz.asciidoc

@@ -164,7 +164,7 @@ directory so that you do not delete important data later on.
 | conf
   | Configuration files including `elasticsearch.yml`
   | $ES_HOME/config
-  | path.conf
+  | <<config-files-location,CONF_DIR>>
 
 | data
   | The location of the data files of each index / shard allocated

docs/reference/setup/install/zip-windows.asciidoc

@@ -234,7 +234,7 @@ directory so that you do not delete important data later on.
 | conf
   | Configuration files including `elasticsearch.yml`
   | %ES_HOME%\config
-  | path.conf
+  | <<config-files-location,CONF_DIR>>
 
 | data
   | The location of the data files of each index / shard allocated

docs/reference/setup/rolling_upgrade.asciidoc

@@ -88,9 +88,8 @@ To upgrade using a zip or compressed tarball:
     overwrite the `config` or `data` directories.
 
 *   Either copy the files in the `config` directory from your old installation
-    to your new installation, or set the environment variable `ES_JVM_OPTIONS`
-    to the location of the `jvm.options` file and use the `-E path.conf=`
-    option on the command line to point to an external config directory.
+    to your new installation, or set the environment variable
+    <<config-files-location,`CONF_DIR`>> to point to a custom config directory.
 
 *   Either copy the files in the `data` directory from your old installation
     to your new installation, or configure the location of the data directory